Resistive circuits of the present invention may be provided at the analogue outputs of digital-to-analogue converter (DAC) circuitry to serve as termination resistors, but may also serve as resistors or termination resistors (e.g. on an IC chip or in an IC-chip package) in general. For example, it is common for integrated circuitry to have outputs which need to be terminated.
By way of background, FIG. 1 presents a schematic view of integrated circuitry 1 comprising a DAC 2. The DAC 2 has two outputs 4 terminated by termination resistors 6, and connected to output terminals or pins 8 as shown. An example typical value for such termination resistors 6 is 50Ω. This typical value will be used as a running example herein for convenience.
Typically, in integrated circuitry, such termination resistors would be implemented on-chip in undoped polysilicon, and two related problems arise.
Firstly, due to process variation, one could expect a 3σ tolerance in the undoped-polysilicon resistor value of e.g. ±20%. Thus, given an ideal value of 500Ω, one might expect a value in the range 40-60Ω. This presents the problem of inaccurate termination and hence reflections—particularly, signal reflections which vary with frequency, leading to inaccuracies in frequency response and delay errors in impulse response, in other words signal waveform distortion. For example, the overall circuitry may need to meet strict scattering-parameter design specifications, e.g. S11 (input port voltage reflection coefficient) specifications, to prevent such reflections satisfactorily. With a design requirement of e.g. >20 GHz bandwidth it is difficult to switch resistors given that such switching circuitry would present its own parasitic capacitance which badly affects S11, and as such it is difficult to trim the resistance value without degrading S11 at high frequencies when the impedance of the parasitic capacitance falls.
Secondly, the resistor has an associated parasitic capacitance. If, for example, the DAC output current swing were to be 10 mA to 30 mA and the two outputs were differential outputs, average current may be 20 mA for each output. This sets current rating; the resistors 6 would need around 100 μm width, with an example design specification on the undoped polysilicon resistor of around 0.2 mA/μm width. This width presents a significant parasitic capacitance which is disadvantageous as it can have a big effect on high-frequency signals as may be output by the DAC 2.
It is desirable to solve some or all of the above problems.